QbsProjectManager: Take the file type into account...

...when rebuilding the project tree.

Consider this product:
CppApplication { files: ["form.ui"] }

Now change it to this:
QtGuiApplication { files: ["form.ui"] }

The qbs file tags will now include "ui", so the Qt Creator
file type must change from "UnknownFileType" to "FormType". Therefore,
we need to replace the respective file node in the tree.

Change-Id: I6aa7e66d1af584e8ded2ffbb9b1215e8fc1135b8
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>